Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and oxygen (O2) is supplied to the cathode. In some systems, the source of the hydrogen is reformate and the source of the oxygen (O2) is air.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load (such as the batteries or the motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (e−) are taken up to form water (H2O).
There are several fuel cell subsystems within a fuel cell system that require a separately controlled source of pressurized air. For example, these fuel cell subsystems include combustors, partial oxidation (POx) reactors, preferential oxidation (PrOx) reactors, the fuel cell stack and/or other fuel cell subsystems. The fuel cell subsystems typically employ mass flow controllers, mass flow sensors and one or more compressors to provide the air.
When two or more fuel cell subsystems require a controlled amount of pressurized air, some conventional fuel cell systems use a compressor for each subsystem. Each compressor is typically controlled based on the desired airflow that is required by the associated fuel cell subsystem. While this control method is accurate and relatively simple from a control standpoint, the duplication of compressors is undesirable from cost, weight and packaging standpoints.
In other conventional fuel cell systems, a single compressor supplies the air to all of the fuel cell subsystems. A controller sums the mass flow requirements for all of the fuel cell subsystems. The controller commands the compressor to provide the summed mass flow requirement. In this fuel cell control system, an overflow valve is typically required to bleed off excess air due to system errors. The transient response of this control method is inherently compromised due to coupling between the fuel cell subsystems. This control system also requires significant rework for any changes in the fuel cell system.
For example, when mass flow-based control is used and five fuel cell subsystems request 1 g/s flow, the controller sums the mass flow rates and attempts to provide 5 g/s. If one of the flow sensors is inaccurate, all of the fuel cell subsystems suffer. If one of the fuel cell subsystems has a faulty mass flow sensor or mass flow controller and the fuel cell subsystem actually achieves 1.5 g/s but requires 1 g/s, each of the other fuel cell subsystems are starved of air. Alternately, if the faulty fuel cell subsystem requests 2 g/s but gets only 1 g/s, all of the other fuel cell subsystems receive too much air. In other words, an error in one fuel cell subsystem causes errors in the delivery of air to all of the other fuel cell subsystems.